The mortar will hit more models only if there are more models available to be hit
In a 25 man T3 unit, a mortar hits all 25, a rock lobber hits 21.
The rock lobber wounds 11.5, the mortar wounds 8.333.
The rock lobber is cheaper and has outperformed the mortar on a full sized infantry regiment.
Shooting at really huge block of troops the helstorm will outperform the mortar, and is not significantly more expensive.
In fact with multiple templates its entirely possible the helstorm will outperform the mortar on small blocks too.
My point is the mortar is a poor and badly balanced piece of equipment, both against alternatives in other books, and alternatives in our own book.